1. Field of The Invention
The invention relates to non-linear signal processing having a transfer characteristic which is adjustably dependent on the amplitude of an input signal.
2. Description of The Related Art
In a digital circuit, such a non-linear signal processing can be realized in a very simple manner by means of a memory operating as a look-up table. However, in an analog circuit, a non-linear circuit having a transfer which is adjustably dependent on the amplitude of an input signal cannot be realized in an obvious manner.
EP-A-0 382 100, corresponding to U.S. Pat. No. 5,089,890, discloses a gamma correction device which comprises a classifying circuit for dividing an input video signal into a plurality of level ranges according to a plurality of signal levels, a computing circuit for computing the region of the video signal within each of the level ranges obtained by the classifying circuit, and a gamma correction control circuit which is arranged to have the gamma correction characteristic thereof controlled, according to each region computed by the computing circuit, to gamma-correct the input video signal and to output the gamma-corrected video signal. This device shows the disadvantage that a period of time is required for computing the region according to the level range of the video signal. Therefore, the gamma-correction cannot be carried out in real time.
DE-A-42 37 420, corresponding to U.S. Pat. No. 5,307,166, discloses an automatic image-tone control circuit and method for controlling image brightness. An input-output characteristic having an arbitrary line graph is obtained by plural gradient adjusting circuits in response to frequencies of appearance of the brightness of the luminance which are detected by a plural brightness frequency detection circuits. Each gradient adjusting circuit comprises an adjusting current generation circuit, and an adding circuit for adding an adjusting current from the corresponding adjusting current generation circuit to other adjusting currents from adjusting current generation circuits of lower ranking gradient adjusting circuit, these other adjusting currents being weighted by factors depending on the difference in rank between the present gradient adjusting circuit and the respective lower ranking gradient adjusting circuit. Each gradient adjusting circuit further comprises a gradient adjusting current generation circuit for multiplying all of the input signal by a factor which depends on an output signal of the corresponding adding circuit. This results in a rather complex circuit with a limited flexibility.